Injection systems and in particular leakage oil-less common rail injection systems require a control element, such as a piezo actuator in the high pressure space for instance. To ensure the working capacity of the piezo actuator also under high pressures of up to 2000 bar, the pressure must also be able to laterally act on the piezo stack or, as applicable, the piezo ceramic body, in order to assist with the expandability of the piezo stack of the piezo actuator.
Such a piezo actuator is described for instance in WO 02/061856 A1. In this context, the ceramic body of this piezo actuator is encased with a polymer or plastic collar. However, a hermetic seal of the ceramic body in respect of the highly pressurized fuel, such as for instance 2000 bar, is hardly or not feasible in the case of the plastics known on the date of filing of the present patent application. Due to an occasional unavoidable electrical conductibility of conventional fuels, as a result of a minimal acid content for instance, it is already possible for voltage sparkovers to occur between the internal electrodes of the piezo actuator even with minimal wetting of the piezo ceramic. In addition, high expansions of the plastic casing occur on the polarity flaws, which intensify this problem. Furthermore, WO 02/061856 describes the use of a filling material between the piezo stack and the polymer or plastic collar. With the described filling material, the problem nevertheless exists in that said filling material can flow into existing spaces or joints if the piezo stack expands and can be destroyed if the piezo stack moves in the opposite direction. The filling material is thus deposited or destroyed during the period of operation of the piezo actuator. The reduced filling material nevertheless specifies that the pressure present on the exterior of the piezo actuator can no longer be efficiently transmitted to the piezo stack.
Furthermore, a solution using a hermetically sealed, metallic sleeve to transmit the pressure to the piezo stack of the piezo actuator is known internally to the applicant, said solution providing an actuator interior space between the piezo stack and the sleeve. In the actuator interior space, a filler, e.g. a silicon oil, is provided to transmit the hydraulic forces onto the side surfaces of the piezo ceramic body. This solution known internally to the applicant is however disadvantageous in that the seal between the piezo stack and the sleeve, at the point at which an electrical pin passage or contact device for contacting and controlling the piezo stack is guided through the seal, does not remain hermetically sealed in the case of longer periods of operation and in particular with high pressures, such as 2000 bar. In the event of a leakage, the risk thus exists that the filler could escape from the actuator interior space. A filler leakage could render the piezo actuator and thus the injection system inoperable.